the evaluation by means of microindentation and

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Adrian BARYLSKI*, Jerzy CYBO*, Joanna MASZYBROCKA*
THE EVALUATION BY MEANS
OF MICROINDENTATION AND SCLEROMETRY
OF CHANGES IN PROPERTIES AFTER ELECTRON
BEAM IRRADIATION OF POLYETHYLENE
GUR 1050
DIAGNOZOWANIE METODĄ MIKROINDENTACJI
I SKLEROMETRII ZMIAN WŁAŚCIWOŚCI
PO NAPROMIENIOWANIU STRUMIENIEM ELEKTRONÓW
POLIETYLENU GUR 1050
Key words:
radiation modification; operational load; polymer deformation; abrasive wear
micromechanism; resistance to wear ratio; hardness; Young’s modulus
Słowa kluczowe:
modyfikacja radiacyjna obciążenie eksploatacyjne; odkształcenie polimeru;
mikromechanizm zużycia ściernego; wskaźnik odporności na zużycie;
twardość; moduł Younga
*
University of Silesia, Department of Materials Science, Faculty of Computer Science and Materials Science, Street Śnieżna 2, PL-41-200 Sosnowiec, Poland, fax. +48323689572, e-mail:
[email protected].
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Summary
With the aim to increase the resistance of GUR 1050 polyethylene to plastic
deformation and abrasive wear, modification through radiation was applied.
Compression tests of up to 300 MPa showed an increase in resistance to
permanent deformation. This has been confirmed with microindentation and
sclerometry tests. Scratch test indicated a lower tendency of the carved material
to chipping than to uplift in all operational deformations. The domination of the
micromechanism of the formation of ridges allows the material to retain a small
decrease in resistance to wear. The parameters of microtests may be used for
the purpose of the evaluation and selection of the polymers used in friction
joints in medicine and technology.
INTRODUCTION AND EXPERIMENTAL PROCEDURE
The operational durability of polymer and metal kinematic systems (e.g.
endoprostheses for human joints) depends mainly on polyethylene resistance to
plastic deformation and abrasive wear [L. 1–8]. Recently, a GUR 1050
polymer has been introduced for the production of medical implants. It is
therefore justified to examine the susceptibility of the material to deformation
as well as the possibility to improve its properties by means of its irradiation
with an electron beam. The GUR 1050 polymer, which has been marked as
BZ50 in its initial (base) state, was subjected to analysis. Tests were conducted
on deformed samples of initial material (BZ50) and on the material irradiated
with an electron beam before radiation (NO50k). The index 50 corresponds to
the tested grade of polymer, letters N and O indicate irradiation and
deformation, and k = 1-4 means the multiplication factor of irradiation with a
dose of 26 kGy. Modification through radiation was performed using a linear
accelerator Elektronika 10/10 (energy of electrons 10 MeV; beam power
10 kW). The samples were compressed with the rate of 5 mm/min at room
temperature. Total compressions of Ztot of 40, 50, 60, 70, and 80% were applied
(three samples were prepared for each compression intended). The compression
Ztot is defined by Ztot = 100·∆h/h0, where h0 is the initial height of the sample, h1
is the height of sample under maximum load, and ∆h = h1 – h0 is the height
reduction due to compression. After reaching the assumed compression, the
sample was immediately unloaded. The load Ptot, corresponding to the applied
compression Ztot, was recorded and the nominal stress, σtot = Ptot/A0 (A0 is the
initial cross-section area of the sample), was calculated. True plastic strain is
defined as ef = ln(h0/hf), where hf means the final height of the samples after
elastic recovery. The recovered elastic deformation component Zel was
determined as a difference between Ztot and Zef, where Zef = (h0-hf)/h0·100%.
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Micromechanical and sclerometric properties of the polymer were
determined with the use of a Micron-Gamma machine equipped with an
additional self-levelling table. This solution ensures that the surface of the
sample and the base of the measuring table are parallel to each other, which
significantly increases the accuracy of the measurements. In microindentation
tests, a Berkovich penetrator was used, load 1N, and at a load time under
maximum pressure of 15s. To determine the hardness H and the elasticity
modulus E, the Oliver-Phare method was applied. The algorithm approximating
the curve of unloading covered an analysis 70% of its range for the elasticplastic contact. The measurement results were averaged for 7 indentations. In
sclerometric tests, the arrowhead of the triangular blade in Berkovich indenter
was set motion-wise. During the scratch test, a nominal power of 2.5 N and a
scratch speed of 90 µm/s were applied. The measurement of the indented
surface A and of the plastic uplift was performed with the use of Taylor
Hobson profilographometer equipped with TalyMap Universal software. This
allowed the authors to determine the abrasive wear micromechanism β as well
as the wear resistance ratio Wβ (1) and (2) (Fig. 1).
β=
1 n A i− B i
∑
n i =1 A i
Wβ =
1
n
(
1
∑ β iA i
n i =1
)
[-]
(1)
[mm-2]
(2)
Fig. 1. A cross-section of a scratch resulting from a scratch test
Rys. 1. Schemat przekroju poprzecznego rysy powstałej podczas scratch testu
RESULTS AND DISCUSSION
The results of the compression test were approximated with a regression
equation (R ≥ 0.97) expressing the stress σtot in deformation function Ztot, Zel, ef,
and then transforming these relations to the following (3) – (5) (Figs. 2–4):
Z tot =
1 
1
πR e 
ln   σ tot −

0.06  w 
2 
ef =
Zel =

1  2
ln
σ tot 
n  πR e

 1

1
(σ tot − 2.5R e )
ln 
0.6  w el

[%]
(3)
[-]
(4)
[%]
(5)
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where:
Re (=21MPa) – the plasticity limit of the GUR 1050 polyethylene,
w (=w0+wk) – the increase in the intensity of the stress with total
unitary deformation of the initial material (w0 =
= 1.78 MPa / % for BZO50) and the irradiated material
(wk = 0.16 k for NO50.k),
k (=1-4)
– multiplication factor of the electron beam irradiation
dose (d = 26kGy),
n (= n0 + nk) – increase in the ratio of the resistance put up by the
material with the increase of plastic deformation of
the initial material (n0 = 3.36 for BZO50) and the
irradiated material (nk = 0.13 k for NO50.k),
7
wel ·10
– the intensity of the stress increase with unitary elastic
deformation (=1.5 and 2.5 MPa / % of the initial
material BZO50 and the irradiated material NO50.k).
Lower values of the total deformation Ztot and true plastic strain ef in the
compression test are visible in the irradiated material, in particular, when the
stress exceeded 50 MPa (Figs. 2 and 3). The undesirable effect of irradiation is
a slight decrease in elastic deformation, which drops by approximately one
percentage point at stresses of σtot ≥100 MPa (Fig. 4).
Fig. 2. Changes in total deformation during
as a function of stress σtot (3)
Rys. 2. Zmiany całkowitej deformacji w funkcji naprężenia σtot (3)
Fig. 3. Changes in true plastic strain
during as a function of stress σtot (4)
Rys. 3. Zmiany rzeczywistego odkształcenia
plastycznego w funkcji naprężenia σtot
(4)
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Fig. 4. Changes in elastic deformation during as a function of stress σtot (5)
Rys. 4. Zmiany odkształcenia sprężystego w funkcji naprężenia σtot (5)
The improvement of the resistance to plastic deformation was confirmed
by hardness and Young’s modulus tests. The H value increases with the k
multiplication factor of the electron beam irradiation. It is observed that an
increase in the degree of plastic deformation ef during compression (which
simulates operational deformation) is accompanied by a linear drop in hardness
(Fig. 5) The changes in H were approximated (taking into account the
measurement taken for BZk samples) by the regression equation (R.≥.0.95), (6):
H = H BZk − I H ⋅ e f [MPa]
(6)
where: HBZk – the hardness of the initial (base) material after being irradiated
with an electron beam k-times (Table 1),
– the intensity of the effect the true plastic strain ef has on
IH
hardness (the change in hardness with a unitary increase of
plastic deformation).
Table 1. The values of the equation (6)
Tabela 1. Wartości równania (6)
k
HBZk [MPa]
IH [MPa]
0
58
2
61
21
4
64
Fig. 5. Changes in the hardness of the GUR 1050 polymer as a function of true plastic strain ef (6)
Rys. 5. Zmiany twardości w polimerze GUR 1050 w funkcji rzeczywistego odkształcenia plastycznego ef (6)
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A similar co-dependence may be observed for the elasticity modulus E. It
is a consequence of the linear co-relation of Young's modulus with hardness (7)
with a co-relation ratio R = 0.96:
[MPa]
(7)
E = 25H − 350
The analysis of the micromechanism and the wear resistance ratio suggests
the increase in the tribological properties of the polymer after a radiation
modification. Parameter β indicates that lower tendency of the carved material to
chip rather than to uplift is maintained in all operational deformations ef This proves
the prevalence of ridging (β0) over the micromechanism of wear through slicing
(β1), (Fig. 6). As a result, a low intensity of decrease in resistance to abrasive
wear Wβ is noted despite the increase in operational deformation ef of the
polyethylene. The increase in resistance Wβ is proportional to the dose of the
irradiation with an electron beam that GUR 1050 is subjected to (Fig. 7).
Fig. 6. Changes in abrasive wear micromechanism as a function of true plastic
strain ef
Rys. 6. Zmiany mikromechanizmu zużycia
ściernego w funkcji rzeczywistego odkształcenia plastycznego ef
Fig.7. Changes in resistance to abrasive wear
ratio as a function of true plastic
strain ef
Rys. 7. Zmiany wskaźnika odporności na zużycie ścierne w funkcji rzeczywistego odkształcenia plastycznego ef
SUMMARY
The analysis of the changes in the deformation values being the result of
operational load showed the increase in the resistance of polyethylene to total
deformation as a result of applied irradiation with an electron beam. The
improvement of its properties has been corroborated by the results of
sclerometric and microindentation microtests. The increase in hardness and
Young's modulus proportional to the dose of irradiation applied have been
found. Scratch test showed the lower tendency of the modified polymer to chip
rather than to uplift. This proves that the abrasive wear may result in the
smaller loss of the mass of the material. After irradiation with an electron beam,
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a significant increase in wear resistance ratio is noted. Increasing operational
deformation reduces the wear resistance of polyethylene.
The parameters of microtests may be used for the purpose of evaluation
and selection of the polymers used in friction joints in medicine and
technology.
The presented results will be subjected to verification in tribological tests
of polymer and metal kinematic systems, which will reflect the conditions in
which an endoprosthesis of a hip joint operates.
REFERENCES
1.
2.
3.
4.
5.
6.
7.
8.
Greer K.W., Hamilton J.V.: Polyethylene Wear in Orthopedics, J&JP, Inc, 1994.
Edidin A.A., Rimnac C.M., Kurtz S.M.: Mechanical behavior, wear surface morphology, and clinical performance of UHMWPE acetabular components after 10
years of implantation, Wear 250(2001) 152–158.
A.A. Edidin, C.M. Rimnac, S.M. Kurtz, Mechanical behaviour, wear surface morphology and clinical performance of UHMWPE acetabular components after 10
years of implantation. Wear 2001 (2005) 152–158.
S.M. Kurtz, O.K. Muratoglu, M. Evans, A.A Eddin, Advances in the processing,
sterylization and crosslinking of ultra high molecular weight polyethylene for total
joint arthoplasty. Biomaterials 20 (1999) 1559–1682.
A.A. Eddin, S.M. Kurtz, The evolution of paradigms for wear in total joint
arthoplasty. The Role of Design. Material and Mechanics (2000) 1–14.
Maszybrocka J., Cybo J., Frąckowiak J.: Change of micromechanical properties of
polyethylene induced by a tribological process in polymer/metal system, Mater Sci
Forum 513 (2006) 363–372.
Duda P., Cybo J., Maszybrocka J., Barylski A.: The morphology, texture and structure orientation of polyethylene formed via work hardening and electron irradiation.
Inż. Mater. 164 (2008) 198–203.
Maszybrocka J., Cybo J.: Changes to the morphology, structure and properties as a
consequence of polyethylene working in a polymer-metal kinematic pair. Materials
Characterization 10 (2009) 1139–1144.
Streszczenie
W celu podwyższenia odporności polietylenu GUR 1050 na odkształcenie
plastyczne i zużycie ścierne zastosowano modyfikację radiacyjną. Badania
w zakresie naprężeń do 300 MPa wykazały wzrost odporności na deformację
trwałą. Potwierdzono to testami mikroindentacyjnymi i sklerometrycznymi.
Scratch test wykazał, że w całym zakresie odkształceń eksploatacyjnych
występuje mniejsza skłonność do wykruszania materiału wyżłobionego niż do
jego plastycznego wypiętrzania. Dominacja mikromechanizmu bruzdowania
pozwala zachować niewielki spadek odporności na zużycie. Parametry
mikrotestów mogą być wykorzystywane do diagnozowania i selekcji polimerów stosowanych w medycznych oraz technicznych węzłach tarcia.
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